New Mexico Geological Society Annual Spring Meeting — Abstracts

The igneous rocks of the Florida Mountains, near Deming, NM, consist of a core complex of syenites, alkali granites, and some quartz monzonite, with lesser amounts of gabbro, diorite, and syenodiorite. These rocks, together with Paleozoic and younger sedimentary rocks, are abundantly cut by Tertiary rhyolitic and andesitic dikes.

The core complex has been extensively studied by several investigators during the past twelve years. Corbitt (1971) and Corbitt and Woodward (1973) assigned a Mesozoic (?) age to these rocks, based on very limited geochronologic data, coupled with their interpretation of an intrusive origin for several syenite-sedimentary rock contacts. Denison (in Brookins, 1974) reported K-Ar and Rb-Sr mineral ages for syenites ranging from 420 to 550 MYBP, although he considered the older ages suspect. Brookins (1974, 1980) reported dates ranging from 400 MYBP for syenites, up to BYBP for certain granites, with most of the ages falling in the early-to-mId-Paleozoic. Clemons (1982). however, has stated that the syenites are Precambrian, reinterpreting several of Corbitt's :lntrusive contacts as faults and unconformities, and explaining the young dates for syenites as being a result of Tertiary resetting relat.ed to volcanic activity.

Our present studies now include Rb-Sr whole rock analyses of some 35 samples of syenite, alkali granite, and gabbro, all of which define a 420 + 20 MYBP isochron. To explain this age as being due to a resetting event demands perturbation of Rb-Sr whole rock systematics to an extent never before demonstrated. We consider this unlikely. Further, our recent field studies indicate the possibility of intrusive or low angle fault contacts at sites wher unconformable contacts have recently been proposed. If these observations are verified by addition field, petrographic, and chemical work, then Paleozoic plutonism is again proposed for some of the core rocks of the Florida Mountains.